Off-line controller integrated circuits for providing the drive signal(s) for the switching device(s) of an off-line converter are known. The off-line converter converts an unregulated DC voltage, such as may be provided by rectifying an AC line voltage, into a regulated DC output voltage. In general, off-line controllers include circuitry for comparing a feedback signal representative of the converter output with a reference signal in order to generate the drive signal(s) for the converter switching device(s). The drive signal(s) adjust the duty cycle of the switch(es) in response to the feedback signal in order to regulate the converter output voltage to a predetermined level. The feedback signal may be the DC output voltage, the output current, or some signal proportional to the output voltage or current and/or transformer isolated from the output.
Some off-line controllers include an undervoltage lockout feature, whereby the line voltage is sensed and the drive signal is inhibited (i.e., forced to a level necessary to keep the converter switching device(s) off) if the line voltage falls below a predetermined level. One way of implementing this feature is to provide a resistor divider, external to the controller, to establish a voltage proportional to the line voltage for comparison to a reference voltage. However, this arrangement requires that the controller include an additional pin for connection to the resistor divider for the purpose of sensing the line voltage.
Often, the controller circuitry is powered by the unregulated DC line voltage at start-up, until the regulated converter output voltage is within a specified range. Connection between the line voltage and the controller for this purpose may be achieved with a series resistor and capacitor arrangement, external to the controller, with the interconnection between the resistor and capacitor connected to a supply voltage, or V.sub.CC, input pin of the controller and the capacitor further connected to ground. Once the converter output voltage rises to a predetermined level, an alternative mechanism provides the supply voltage V.sub.CC to the controller and maintains a predetermined charge on the external supply voltage capacitor, such as an additional secondary winding on the converter transformer. This alternative current for charging the supply voltage capacitor is sometimes referred to as a bootstrap current.